Consider the following algorithm for topological sort given in my textbook:

Input: A digraph G with n vertices
Output: A topological ordering v1,v2...vn of G, or the non-existence thereof.
S is an empty stack
for each vertex u in G do
incount(u) = indeg(u)
if incount(u) == 0 then
S.push(u)
i = 1
while S is non-empty do
u = S.pop()
set u as the i-th vertex vi
i ++
for each vertex w forming the directed edge (u,w) do
incount(w) --
if incount(w) == 0 then
S.push(w)
if S is empty then
return "G has a dicycle"

I tried implementing the algorithm word-for-word but found that it always complained of a dicycle, whether the graph was acyclic or not. Then, I discovered that the last 2 lines don't fit in correctly. The while loop immediately prior to it exits when S is empty. So, each time, it is assured that the if condition will hold true.

How can I correct this algorithm to properly check for a dicycle?

Edit:

Presently, I'm simply skirting the S problem, by checking the value of i at the end: